DETERMINANTS OF MUSCULOSKELETAL FLEXIBILITY - VISCOELASTIC PROPERTIES, CROSS-SECTIONAL AREA, EMG AND STRETCH TOLERANCE

Cross-sectional area, stiffness, viscoelastic stress relaxation, stret ch tolerance and EMG activity of the human hamstring muscle group were examined in endurance-trained athletes with varying flexibility. Subj ects were defined as tight (n=10) or normal (n=8) based on a clinical toe-touch test. Cross-sectional area was computed from magnetic resona nce imagining (MRI) images. Torque (Nm) offered by the hamstring muscl e group, electromyographic (EMG) activity, knee joint angle and veloci ty were continuously monitored during two standardized stretch protoco ls. Protocol 1 consisted of a slow stretch at 0.087 rad/s (dynamic pha se) to a pre-determined final angle followed by a 90-s static phase. I n the dynamic phase final angle and stiffness was lower in tight (28.0 +/-2.9 Nm/rad) than normal subjects (54.9+/-6.5 Nm/rad), P<0.01. In th e static phase tight subjects had lower peak (15.4+/-1.8 Nm) and final torque (10.8+/-1.6 Nm) than normal subjects (31.6+/-4.1 Nm, 24.1+/-3. 7 Nm, respectively)(P<0.01), but torque decline was similar. Protocol 2 consisted of a slow stretch to the point of pain and here tight subj ects reached a lower maximal angle torque, stiffness and energy than n ormal subjects (P<0.01). On the other hand, stiffness was greater in t ight subjects in the common range (P<0.01). Cross-sectional area of th e hamstring muscles and EMG activity during the stretch did not differ between the groups. However, lateral hamstring cross-sectional area w as positively related to mid-range stiffness (P<0.05), but inversely r elated to final stiffness, peak torque and the toe-touch test (P<0.01) . Final angle and peak torque in protocol 1 combined to improve the pr edictability of the toe-touch test (R-2=0.77, P<0.001). These data sho w that the toe-touch test is largely a measure of hamstring flexibilit y. Further, subjects with a restricted joint range of movement on a cl inical toe-touch test have stiffer hamstring muscles and a lower stret ch tolerance.